The invention relates generally to the electrical system of an automotive vehicle and, more particularly, to control circuitry for controlling the supply of power to an electrically heatable windshield of a quantity that provides deicing of said windshield.
Numerous electrical systems have been developed for deicing automotive windshields and backlites of the type which employ heating elements such as thin film metalization or a pattern of resistive heating wires embedded in the window material, which exhibit good optical transparency. A principal requirement of these systems is to supply power to the heating wires that, within safe limits of the window material, can provide a relatively rapid deicing of the window to temperatures well below freezing. This has been found to require the generation of voltages substantially higher than the usual regulated battery voltage. A second important requirement is to supply such heating power without adversely affecting the vehicle's battery charge requirements and electrical operation. A number of systems have employed high voltage auxiliary alternators for this purpose, which operate substantially independent of the existing automotive electrical system. While these systems can be designed to directly meet the requirements of good deicing performance, they are not a viable solution for the mass automotive market because of space and weight restrictions and the relatively high cost of such added alternator equipment.
In other type systems the standard vehicle alternator is employed to supply power to the resistive heating elements. In such cases the existing automotive circuit must be modified to generate the relatively high power required for the deicing function, and also, as possible, maintain the requisite charge on the vehicle battery and fulfill the normal automotive load requirements. In one exemplary system of this type the alternator is for a short period disconnected from the standard automotive circuit including the battery, load and voltage regulator and is connected to the resistive heating elements for the purpose of deicing. During this period an unregulated alternator can supply voltages substantially higher than the regulated battery voltage, while the battery supplies the normal automotive load. At the end of the period, the length of which may be determined by a temperature sensor, the alternator is reconnected to the regulator and standard load circuit. While these systems may achieve a reasonably good deicing performance, they have limitations in respect to maintaining at all times the best balance of energy requirements for the battery and load circuitry, and also of generating the safe as well as high voltages necessary for deicing.